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Commit eb619fdb authored by Jens Axboe's avatar Jens Axboe
Browse files

blk-mq: fix issue with shared tag queue re-running 



This patch attempts to make the case of hctx re-running on driver tag
failure more robust. Without this patch, it's pretty easy to trigger a
stall condition with shared tags. An example is using null_blk like
this:

modprobe null_blk queue_mode=2 nr_devices=4 shared_tags=1 submit_queues=1 hw_queue_depth=1

which sets up 4 devices, sharing the same tag set with a depth of 1.
Running a fio job ala:

[global]
bs=4k
rw=randread
norandommap
direct=1
ioengine=libaio
iodepth=4

[nullb0]
filename=/dev/nullb0
[nullb1]
filename=/dev/nullb1
[nullb2]
filename=/dev/nullb2
[nullb3]
filename=/dev/nullb3

will inevitably end with one or more threads being stuck waiting for a
scheduler tag. That IO is then stuck forever, until someone else
triggers a run of the queue.

Ensure that we always re-run the hardware queue, if the driver tag we
were waiting for got freed before we added our leftover request entries
back on the dispatch list.

Reviewed-by: default avatarBart Van Assche <bart.vanassche@wdc.com>
Tested-by: default avatarBart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: default avatarMing Lei <ming.lei@redhat.com>
Reviewed-by: default avatarOmar Sandoval <osandov@fb.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent e454d122

block/blk-mq-debugfs.c

+0 −1
Original line number Diff line number Diff line
@@ -179,7 +179,6 @@ static const char *const hctx_state_name[] = { 
	HCTX_STATE_NAME(STOPPED),

	HCTX_STATE_NAME(TAG_ACTIVE),

	HCTX_STATE_NAME(SCHED_RESTART),

	HCTX_STATE_NAME(TAG_WAITING),

	HCTX_STATE_NAME(START_ON_RUN),

};

#undef HCTX_STATE_NAME

block/blk-mq.c

+48 −37
Original line number Diff line number Diff line
@@ -998,41 +998,55 @@ bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx, 
	return rq->tag != -1;

}



static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode, int flags,

				void *key)

static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode,

				int flags, void *key)

{

	struct blk_mq_hw_ctx *hctx;



	hctx = container_of(wait, struct blk_mq_hw_ctx, dispatch_wait);



	list_del(&wait->entry);

	clear_bit_unlock(BLK_MQ_S_TAG_WAITING, &hctx->state);

	list_del_init(&wait->entry);

	blk_mq_run_hw_queue(hctx, true);

	return 1;

}



static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx *hctx)

static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx **hctx,

				     struct request *rq)

{

	struct blk_mq_hw_ctx *this_hctx = *hctx;

	wait_queue_entry_t *wait = &this_hctx->dispatch_wait;

	struct sbq_wait_state *ws;



	if (!list_empty_careful(&wait->entry))

		return false;



	spin_lock(&this_hctx->lock);

	if (!list_empty(&wait->entry)) {

		spin_unlock(&this_hctx->lock);

		return false;

	}



	ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);

	add_wait_queue(&ws->wait, wait);



	/*

	 * The TAG_WAITING bit serves as a lock protecting hctx->dispatch_wait.

	 * The thread which wins the race to grab this bit adds the hardware

	 * queue to the wait queue.

	 * It's possible that a tag was freed in the window between the

	 * allocation failure and adding the hardware queue to the wait

	 * queue.

	 */

	if (test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state) ||

	    test_and_set_bit_lock(BLK_MQ_S_TAG_WAITING, &hctx->state))

	if (!blk_mq_get_driver_tag(rq, hctx, false)) {

		spin_unlock(&this_hctx->lock);

		return false;



	init_waitqueue_func_entry(&hctx->dispatch_wait, blk_mq_dispatch_wake);

	ws = bt_wait_ptr(&hctx->tags->bitmap_tags, hctx);

	}



	/*

	 * As soon as this returns, it's no longer safe to fiddle with

	 * hctx->dispatch_wait, since a completion can wake up the wait queue

	 * and unlock the bit.

	 * We got a tag, remove ourselves from the wait queue to ensure

	 * someone else gets the wakeup.

	 */

	add_wait_queue(&ws->wait, &hctx->dispatch_wait);

	spin_lock_irq(&ws->wait.lock);

	list_del_init(&wait->entry);

	spin_unlock_irq(&ws->wait.lock);

	spin_unlock(&this_hctx->lock);

	return true;

}
@@ -1041,6 +1055,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, 
{

	struct blk_mq_hw_ctx *hctx;

	struct request *rq, *nxt;

	bool no_tag = false;

	int errors, queued;



	if (list_empty(list))
@@ -1060,22 +1075,15 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, 
		if (!blk_mq_get_driver_tag(rq, &hctx, false)) {

			/*

			 * The initial allocation attempt failed, so we need to

			 * rerun the hardware queue when a tag is freed.

			 * rerun the hardware queue when a tag is freed. The

			 * waitqueue takes care of that. If the queue is run

			 * before we add this entry back on the dispatch list,

			 * we'll re-run it below.

			 */

			if (!blk_mq_dispatch_wait_add(hctx)) {

				if (got_budget)

					blk_mq_put_dispatch_budget(hctx);

				break;

			}



			/*

			 * It's possible that a tag was freed in the window

			 * between the allocation failure and adding the

			 * hardware queue to the wait queue.

			 */

			if (!blk_mq_get_driver_tag(rq, &hctx, false)) {

			if (!blk_mq_dispatch_wait_add(&hctx, rq)) {

				if (got_budget)

					blk_mq_put_dispatch_budget(hctx);

				no_tag = true;

				break;

			}

		}
@@ -1140,10 +1148,10 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, 
		 * it is no longer set that means that it was cleared by another

		 * thread and hence that a queue rerun is needed.

		 *

		 * If TAG_WAITING is set that means that an I/O scheduler has

		 * been configured and another thread is waiting for a driver

		 * tag. To guarantee fairness, do not rerun this hardware queue

		 * but let the other thread grab the driver tag.

		 * If 'no_tag' is set, that means that we failed getting

		 * a driver tag with an I/O scheduler attached. If our dispatch

		 * waitqueue is no longer active, ensure that we run the queue

		 * AFTER adding our entries back to the list.

		 *

		 * If no I/O scheduler has been configured it is possible that

		 * the hardware queue got stopped and restarted before requests
@@ -1155,8 +1163,8 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, 
		 *   returning BLK_STS_RESOURCE. Two exceptions are scsi-mq

		 *   and dm-rq.

		 */

		if (!blk_mq_sched_needs_restart(hctx) &&

		    !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state))

		if (!blk_mq_sched_needs_restart(hctx) ||

		    (no_tag && list_empty_careful(&hctx->dispatch_wait.entry)))

			blk_mq_run_hw_queue(hctx, true);

	}
@@ -2020,6 +2028,9 @@ static int blk_mq_init_hctx(struct request_queue *q, 


	hctx->nr_ctx = 0;



	init_waitqueue_func_entry(&hctx->dispatch_wait, blk_mq_dispatch_wake);

	INIT_LIST_HEAD(&hctx->dispatch_wait.entry);



	if (set->ops->init_hctx &&

	    set->ops->init_hctx(hctx, set->driver_data, hctx_idx))

		goto free_bitmap;

include/linux/blk-mq.h

+2 −3
Original line number Diff line number Diff line
@@ -181,8 +181,7 @@ enum { 
	BLK_MQ_S_STOPPED	= 0,

	BLK_MQ_S_TAG_ACTIVE	= 1,

	BLK_MQ_S_SCHED_RESTART	= 2,

	BLK_MQ_S_TAG_WAITING	= 3,

	BLK_MQ_S_START_ON_RUN	= 4,

	BLK_MQ_S_START_ON_RUN	= 3,



	BLK_MQ_MAX_DEPTH	= 10240,